Method for producing propylene oxide

ABSTRACT

A process for preparing propylene oxide comprises at least the following steps:
         (a) propene is reacted with a hydroperoxide in a solvent in the presence of a zeolite catalyst to give a mixture (M 0 ), where the mixture (M 0 ) comprises propylene oxide, solvent, unreacted propene, unreacted hydroperoxide and oxygen,   (b) the propylene oxide is separated from the mixture (M 0 ) so as to give a mixture (M 1 ) which comprises unreacted propene and oxygen,   (c) the mixture (M 1 ) is brought into contact with a liquid medium comprising at least a solvent to give the mixtures (M 2 ) and (M 3 ), where the mixture (M 3 ) comprises part of the unreacted propene and oxygen and the mixture (M 2 ) comprises solvent and residual propene, and    the mixture (M 3 ) which has been separated off and comprises unreacted propene and oxygen has a ratio of oxygen to propene such that the mixture (M 3 ) is not ignitable, and    the mixture (M 2 ) is fed to at least one reaction of propene with hydroperoxide.

The present invention relates to a process in which propylene oxide isprepared from a hydroperoxide, preferably hydrogen peroxide, andpropene, where part of the unreacted propene is recovered by means of anappropriate procedure and is returned to a reaction of propene withhydroperoxide.

In processes in which mixtures of an alkene and oxygen are obtained, itis frequently desirable to separate all or some of the alkene from thismixture and, for reasons of process economics, return it to a stage ofthe process. A problem which arises in such processes is the formationof ignitable mixtures which of course have to be avoided under allcircumstances for safety reasons.

A process in which this problem occurs is the preparation of propyleneoxide, an important intermediate in the chemical industry, from propeneand hydrogen peroxide. During the work-up of the product in thisprocess, unreacted propene is preferably separated off from the productmixture from the epoxidation and is reused as starting material in theprocess. When the propene is separated off from the product mixture, itis frequently accompanied by other low-boiling components which arelikewise present in the product mixture, including oxygen. The oxygen isusually concentrated in the propene and reaches a concentration whichleads to an ignitable mixture of propene and oxygen. This represents asafety risk in the separation process which must not be underestimated.

To solve this problem, EP-B 0 719 768 proposes carrying out theseparation of propene from the low-boiling mixture in an absorptionzone. In this, an inert gas, preferably methane, is introduced to such aconcentration that the oxygen which is likewise present in thelow-boiling mixture is diluted to a concentration at which the mixtureis no longer in the ignitable range. Furthermore, this procedurerequires a liquid absorption medium by means of which the propene isscrubbed from the low-boiling fraction to be fed to the absorption zone.

Accordingly, two additional components, viz. the inert gas and theliquid absorption medium, have to be used in the process just describedfor the epoxidation of propene using hydrogen peroxide in order toseparate off or recover unreacted propene while avoiding the formationof flammable gas mixtures.

This process is therefore encumbered by increased costs and increasedcomplication in terms of apparatus for the use of the additionalcomponents necessary in this process for safety reasons.

It is an object of the present invention to provide a process forpreparing propylene oxide in which it is possible to recover at leastpart of the unreacted propene from a gaseous mixture comprising oxygenand unreacted propene in a simple and safe manner in order to reuse itas starting material in a stage of the process.

We have found that this object is achieved by a process for preparingpropylene oxide, which comprises at least the following steps:

-   -   (a) propene is reacted with a hydroperoxide in a solvent in the        presence of a zeolite catalyst to give a mixture (M0), where the        mixture (M0) comprises propylene oxide, solvent, unreacted        propene, unreacted hydroperoxide and oxygen,    -   (b) the propylene oxide is separated from the mixture (M0) so as        to give a mixture (M1) which comprises unreacted propene and        oxygen,    -   (c) the mixture (M1) is brought into contact with a liquid        medium comprising at least a solvent to give the mixtures (M2)        and (M3), where the mixture (M3) comprises part of the unreacted        propene and oxygen and the mixture (M2) comprises solvent and        residual propene, and    -    the mixture (M3) which has been separated off and comprises        unreacted propene and oxygen has a ratio of oxygen to propene        such that the mixture (M3) is not ignitable, and    -    the mixture (M2) is fed to at least one reaction of propene        with hydroperoxide.

The separation of a mixture into at least two fractions can, for thepurposes of the invention, in principle be carried out by all suitablemethods.

For the purposes of the present invention, the separation of anessentially liquid mixture is preferably achieved by means of at leastone distillation column.

On the other hand, the complete or partial separation of at least onegaseous component from an essentially gaseous mixture is, for thepurposes of the present invention, preferably carried out using an“absorption column”.

In this, the essentially gaseous mixture flowing through the column isscrubbed, generally in countercurrent, with a liquid medium (absorptionmedium), so that the desired gas component is absorbed completely orpartially by the absorption medium and leaves the column together withthe absorption medium. The gas component(s) which has/have not beenabsorbed can leave the column via a further suitable device.

In the process of the present invention, the desired gas component to beabsorbed by the liquid medium is propene. The amount of propene which isabsorbed by the liquid medium (absorption medium) during scrubbing ofthe gas can be controlled via all parameters which appear suitable forthis purpose to a person skilled in the art, for example via the amountof absorption medium or the pressure in the absorption column which isin a range from 0.5 to 3 bar, preferably in the vicinity of atmosphericpressure. Control via the temperature prevailing in the absorptioncolumn, which is in the range from 0 to 60° C., preferably in the rangefrom 25 to 40° C., is also possible.

For the purposes of the invention, the parameter used for controllingthe amount of propene which is absorbed by the liquid medium (absorptionmedium) during scrubbing of the gas is preferably the amount ofabsorption medium used.

Preference is given to only part of the propene present in a gaseousmixture, known as the residual propene, being absorbed by the absorptionmedium in the absorption column or columns, so that propene is stillpresent in the gaseous mixture remaining after the scrubbing step.

The liquid medium (absorption medium) used for absorption in the processof the present invention can in principle be any liquid medium suitablefor the absorption of propene. Such media include, for example, allsolvents which are known to those skilled in the art and are suitablefor this purpose. Accordingly, it is possible to use, for example, thefollowing solvents as absorption media:

-   -   water,    -   alcohols, preferably lower alcohols, more preferably alcohols        having less than 6 carbon atoms, for example methanol, ethanol,        propanols, butanols, pentanols,    -   diols or polyols, preferably those having less than 6 carbon        atoms,    -   ethers such as diethyl ether, tetrahydrofuran, dioxane,        1,2-diethoxyethane, 2-methoxyethanol,    -   esters such as methyl acetate or butyrolactone,    -   amides such as dimethylformamide, dimethylacetamide,        N-methylpyrrolidone,    -   ketones such as acetone,    -   nitriles such as acetonitrile    -   or mixtures of two or more of the compounds mentioned.

The absorption medium used for the purposes of the invention ispreferably a liquid medium arising from a preceding process step. Thishas to be able to absorb at least part of the unreacted propene from thegaseous mixture (M1) with which it is brought into contact in theabsorption column during scrubbing of the gas. The liquid medium inquestion particularly preferably comprises at least one solvent used inthe process of the present invention.

For the purposes of the invention, the mixture (M1) comprising unreactedpropene and oxygen is, in step (c), brought into contact with a liquidmedium (absorption medium) comprising at least a solvent to give themixtures (M2) and (M3).

-   -   In this process step, unreacted propene is removed from the        mixture (M1) by the absorption medium and is subsequently        returned to the process of the present invention. The amount of        unreacted propene removed in this step is limited by the        requirement that the mixture (M3), which remains, must not        become ignitable.

Furthermore, it is important from a safety point of view that themixture (M3) which leaves the absorption column via a suitable outletfacility and comprises the propene which has not been absorbed by theabsorption medium together with oxygen has a ratio of oxygen to propeneand any further combustible components present in (M3) which is suchthat the mixture (M3) is not ignitable.

For the purposes of the present invention, the term “not ignitable”means that the composition of the mixture (M3) has to be chosen so thatit lies outside the ignition limits (outside the ignitable range) underthe process conditions under which it is separated off, so that themixture (M3) can be handled without it igniting. According to thedefinition of the Berufsgenossenschaft Chemie BGR 104 part 1 explosionprotection regulations, section B, item 9, the term “ignitable mixture”refers to “a mixture of gases and vapors with one another or with mistsor dusts in which ignition results in a self-propagating reaction”.

Basically, the ignition limits are the lower and upper limitingconcentrations of a combustible gas or vapor in admixture with air (oranother oxygen-containing gas) between which the gas (vapor)/air mixturecan be ignited by heating (ignition temperature) or by means of a spark.The ignition limits are dependent on pressure and temperature. They arespecified as concentration of the combustible gas, vapor or oxygen in %by volume or g/m³ for an initial state of 1 013 mbar and 20° C.

The ignition limits of a mixture depend essentially on the compositionof its main components. Listings which may be mentioned by way ofexample of safety parameters of combustible gases and vapors may befound in the following reference works: Coward & Jones, US Bureau ofMines Bull. 503 (1952); Nabert & Schön, Sicherheitstechnische Kennzahlenbrennbarer Gase und Dämpfe, Deutscher Eichverlag, Braunschweig (1963).

The present invention therefore also provides a process as describedabove in which the concentration of oxygen in the mixture (M3) is lessthan 12% by volume, preferably less than 11% by volume, particularlypreferably less than 10% by volume.

For the purposes of the present invention, it is of course also possibleto use propene containing up to 10% by weight of hydrocarbons other thanpropene.

For example, the propene used can contain up to 10% by weight ofpropane, ethane, ethylene, butane or butenes, either individually or asa mixture of two or more thereof.

Accordingly, the present invention also provides a process as describedabove in which the propene used contains up to 10% by weight of otherhydrocarbons.

Step (a) of the above described process of the present invention isgenerally carried out in a main reactor (1), preferably a shell-and-tubereactor. For the purposes of the invention, it is advantageous for themolar ratio of propene to hydroperoxide in step (a) of the process ofthe present invention to be in a range from 0.85 to 5, preferably from0.9 to 2, particularly preferably from 0.9 to 1.2.

In principle, all hydroperoxides known to those skilled in the art canbe used for the purposes of the present invention. Details regarding thepreparation of hydroperoxides or the preferred hydroperoxides may befound in DE-A 19835907.1.

However, the process of the present invention is preferably carried outusing hydrogen peroxide as hydroperoxide.

Furthermore, the conversion of hydrogen peroxide in step (a) ispreferably in a range from 70 to 99%, preferably in a range from 75 to98%, particularly preferably in a range from 80 to 95%.

Accordingly, the present invention also provides a process as describedabove in which the hydroperoxide is hydrogen peroxide and the conversionof hydrogen peroxide in step (a) is in the range from 80 to 95%.

For the purposes of the present invention, it is in principle possibleto use all solvents which appear suitable to a person skilled in theart. For example, solvents used may be

-   -   water,    -   alcohols, preferably lower alcohols, more preferably alcohols        having less than 6 carbon atoms, for example methanol, ethanol,        propanols, butanols, pentanols,    -   diols or polyols, preferably those having less than 6 carbon        atoms,    -   ethers such as diethyl ether, tetrahydrofuran, dioxane,        1,2-diethoxyethane, 2-methoxyethanol,    -   esters such as methyl acetate or butyrolactone,    -   amides such as dimethylformamide, dimethylacetamide,        N-methylpyrrolidone,    -   ketones such as acetone,    -   nitriles such as acetonitrile    -   or mixtures of two or more of the compounds mentioned.

Methanol is preferably used as solvent for the purposes of theinvention.

As zeolite catalysts in step (a) of the process of the presentinvention, it is in principle possible to use all zeolite catalystsknown to those skilled in the art for such a reaction.

Preference is given to using zeolites in which iron, titanium, vanadium,chromium, niobium or zirconium is present.

Specific examples are titanium-, germanium-, tellurium-, vanadium-,chromium-, niobium- or zirconium-containing zeolites having a pentasilzeolite structure, in particular the types which are assigned on thebasis of their X-ray diffraction patterns to the ABW, ACO, AEI, AEL,AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC,APD, AST, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE,CAN, CAS, CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT,DOH, DON, EAB, EDI, EMT, EPI, ERI, ESV, EUO, FAU, FER, GIS, GME, GOO,HEU, IFR, ISV, ITE, JBW, KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN,MAZ, MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW,MWW, NAT, NES, NON, OFF, OSI, PAR, PAU, PHI, RHO, RON, RSN, RTE, RTH,RUT, SAO, SAT, SBE, SBS, SBT, SFF, SGT, SOD, STF, STI, STT, TER, THO,TON, TSC, VET, VFI, VNL VSV, WIE, WEN, YUG, ZON structures and to mixedstructures derived from two or more of the abovementioned structures. Itis also conceivable for further titanium-containing zeolites having thestructure ITQ-4, SSZ-24, TTM-1, UTD-1, CIT-1 or CIT-5 to be used in theprocess of the present invention. Further titanium-containing zeoliteswhich may be mentioned are those having the ZSM-48 or ZSM-12 structure.

In the process of the present invention, preference is given to using Tizeolites having an MFI, MEL or mixed MFI/MEL structure. Further specificexamples of preferred zeolites are the Ti-containing zeolite catalystswhich are generally referred to as “TS-1”, “TS-2”, “TS-3”, and also Tizeolites having a framework structure isomorphous with β-zeolite.

The present invention therefore also provides a process as describedabove in which the zeolite catalyst is a titanium silicalite catalyst,in particular a titanium silicalite catalyst having the TS-1 structure.

Further details regarding the catalysts which can be used, in particularzeolites, may be found in DE-A 1010139.2.

The mixture (M0) arising from step (a) consists essentially of thefollowing components: propylene oxide as desired process product,solvent, unreacted propene, unreacted hydroperoxide and oxygen.

Further components which may be present in the mixture (M0) are waterand, depending on the propane or hydrocarbon content of the propeneused, propane and further hydrocarbons, where the compounds referred toas “hydrocarbons” are of course different from propene.

In a further step (b), propylene oxide is separated from the mixture(M0) resulting from step (a) of the process of the present invention soas to give a mixture (M1) comprising unreacted propene and oxygen.

The separation of propylene oxide from the mixture (M0) and the furtherwork-up of the other components present in (M0) is, for the purposes ofthe present invention, preferably carried out according to one of thefollowing two process variants, viz. variant 1 and variant 2.

These two variants, which are schematically shown in FIG. 1 (variant 1)and FIG. 2 (variant 2), are described in detail below.

Variant 1:

A preferred method (variant 1, FIG. 1) of separating off the propyleneoxide comprises fractionating the product mixture from the main reactor(1) in a column (2) which directly follows the main reactor (1) to givea top fraction and a bottom fraction. The bottom fraction from thecolumn (2), viz. mixture (M0′), comprises solvent, unreactedhydroperoxide and water. The top fraction from this column (2) comprisesmostly propylene oxide, unreacted propene and oxygen and is passed to afurther column (5).

The top fraction from this column (2) may further comprise small amountsof solvent. In the process of the present invention, the top fractionfrom the column (2) preferably contains less than 60% by weight,particularly preferably less than 50% by weight, of solvent.

In column (5), the propylene oxide is taken off via the bottom fractionand, if appropriate, subjected to further work-up steps in which thepropylene oxide is separated off from the further components present inthe bottom fraction, e.g. solvent and water, and purified.

In the work-up of the bottom fraction from column (5), it is alsopossible to recover the solvent and reuse it in the process. Therecovered solvent is preferably returned to step (a).

The top fraction from the column (5), viz. the mixture (M1), consistsessentially of unreacted propene and oxygen and is passed to theabsorption column (3) for further work-up.

In the absorption column (3), the gaseous mixture (M1) is scrubbed withpart or all of the mixture (M0′), viz. the liquid bottom fraction fromthe column (2). Here, the mixture (M0′) or part thereof, which comprisessolvent, unreacted hydroperoxide and water, serves as absorption mediumfor part of the unreacted propene present in the mixture (M1).

In the process of the present invention, the amount of unreacted propeneseparated from the mixture (M1) can be controlled via the amount ofliquid mixture (M0′) used for scrubbing the gaseous mixture (M1).

The amount of the mixture (M0′) is preferably set by means ofappropriate control systems known to those skilled in the art, which maycomprise, for example, at least one bypass or valve system.

Thus, mixture (M0′) can, if required, be divided into two or morefractions. Preference is given to using at least one of these fractionsin the absorption column as absorption medium (liquid medium) forscrubbing the gaseous mixture (M1).

The fraction of the mixture (M0′) which is not fed to the absorptioncolumn can be combined, in its entirety or in part, with the liquidmixture leaving the absorption column (3), in which the part of thepropene absorbed from mixture (M1) in the gas scrubbing step (residualpropene) is present, to give a mixture (M2).

The scrubbing of the gas in the absorption column (3) thus removes partof the unreacted propene from the mixture (M1). Accordingly, the mixture(M2) coming from this process step comprises residual propene and thecomponents of the absorption medium, solvent, water and unreactedhydrogen peroxide.

The gaseous mixture (M3) which likewise comes from this process step andleaves the absorption column (3) through its own outlet device thereforecomprises the part of the unreacted propene which is not absorbed,together with oxygen.

The conditions in the absorption column (3) have to be chosen so thatunreacted propene and oxygen are present in the gaseous mixture (M3)leaving the absorption column (3) in such a ratio that the mixture isnot ignitable.

Furthermore, it is also possible for small amounts of other volatilecomponents to be present in addition to unreacted propene and oxygen inthe mixture (M3).

The mixture (M2) is fed to at least one further reaction of propene withhydroperoxide in an after-reactor (4), preferably a shell-and-tubereactor.

In the after-reactor (4), mixture (M2) is once again admixed with suchan amount of propene that the unreacted hydroperoxide still present inthe mixture (M2) is almost completely reacted with propene to formpropylene oxide, giving a mixture (M4).

The crude output from the after-reactor (4) accordingly comprisespropylene oxide, solvent, water, unreacted propene and less than 500 ppmof unreacted hydroperoxide.

The mixture (M4) can subsequently be worked up further to separate offthe desired product propylene oxide. All or part of this work-up can inprinciple be carried out in a further procedure separate from theprocess described above.

However, for the purposes of the present invention, the mixture (M4) ispreferably transferred in its entirety to column (5) and there combinedwith the top fraction from column (2).

The mixture in column (5) is processed as described above.

Accordingly, the present invention also provides a process as describedabove in which the mixture (M2) further comprises hydroperoxide and is,in the further steps (d) and (e) following the step (c), fed to areaction of propene with hydroperoxide, as follows:

-   -   (d) the mixture (M2) is admixed with further propene so that the        propene reacts with most of the unreacted hydroperoxide present        in the mixture (M2) to form propylene oxide, giving a mixture        (M4), and    -   (e) the mixture (M4) is returned to the separation of step (b).

Variant 2:

A further preferred method (variant 2, FIG. 2) of separating propyleneoxide from the product mixture from the main reactor (1) comprisestransferring the product mixture into a column (6) which directlyfollows the main reactor (1) and in which the mixture (M0) is separatedinto a top fraction and a bottom fraction.

The bottom fraction from column (6), which comprises propylene oxide,solvent and unreacted hydroperoxide and water, is passed to furtherwork-up steps to work it up further and separate off the propyleneoxide.

The solvent present in this bottom fraction is recovered in the courseof the work-up.

The top fraction from column (6), viz. the mixture (M1), consistsessentially of unreacted propene and oxygen.

Mixture (M1) is transferred into the subsequent absorption column (7).In this, the gaseous mixture (M1) is scrubbed with all or part of thesolvent recovered from the bottom fraction from column (6) so as to givea gaseous mixture (M3) and a liquid mixture (M2). Consequently, therecovered solvent serves as absorption medium for the unreacted propenepresent in mixture (M1) in this variant of the process.

The gaseous mixture (M3) comprises oxygen together with the part of theunreacted propene in mixture (M1) which has not been absorbed by thesolvent. The liquid mixture (M2) accordingly comprises the solventtogether with the propene which it has absorbed (residual propene).

In this variant of the process of the present invention, the amount ofpropene which is absorbed by the absorption medium during scrubbing ofthe gaseous mixture (M1) can thus be controlled via the amount ofsolvent which is used as absorption medium in the absorption column (7).

In this process step, unreacted propene is removed from the mixture (M1)by means of the absorption medium and is subsequently returned to theprocess of the present invention. The amount of unreacted propeneremoved in this step is limited by the requirement that the remainingmixture (M3) must not become ignitable.

The conditions in the absorption column (7) are therefore chosen so thatthe propene-and-oxygen containing mixture (M3) which leaves theabsorption column via its own outlet facility is not ignitable.

Mixture (M2) is discharged from the absorption column, if appropriateworked up, and then returned to step (a) of the process.

In the process of the present invention, the mixture (M2) is worked upif appropriate and returned to step (a) of the process.

The present invention therefore also provides a process as describedabove in which the mixture (M2) is fed to a reaction of propene withhydroperoxide in a further step (f) following step (c), as follows:

-   -   (f) the mixture (M2) is worked up if appropriate and returned to        step (a).        List of Reference Numerals

-   FIG. 1: 1 Main reactor (1)

-    2 Column (2)

-    3 Absorption column (3)

-    4 After-reactor (4)

-    5 Column (5)

-   FIG. 2: 1 Main reactor (1)

-    6 Column (6)

-    7 Absorption column (7)    List of abbreviations

-   P: Propene

-   H: H₂O₂

-   M: Methanol

-   RM: Recycled methanol from work-up

-   A: Offgas

-   Z: To further work-up

1. A process for preparing propylene oxide, which comprises at least thefollowing steps: (a) propene is reacted with a hydroperoxide in asolvent in the presence of a zeolite catalyst to give a mixture (M0),where the mixture (M0) comprises propylene oxide, solvent, unreactedpropene, unreacted hydroperoxide and oxygen, (b) the propylene oxide isseparated from the mixture (M0) so as to give a mixture (M1) whichcomprises unreacted propene and oxygen, (c) the mixture (M1) is broughtinto contact with a liquid medium comprising at least a solvent to givethe mixtures (M2) and (M3), where the mixture (M3) comprises part of theunreacted propene and oxygen and the mixture (M2) comprises solvent andresidual propene, and where the mixture (M3) which has been separatedoff and comprises unreacted propene and oxygen has a ratio of oxygen topropene such that the mixture (M3) is not ignitable, and where themixture (M2) is fed to at least one reaction of propene withhydroperoxide.
 2. The process as claimed in claim 1, wherein theconcentration of oxygen in the mixture (M3) is less than 12% by volume.3. The process as claimed in claim 1, wherein the mixture (M2) furthercomprises hydroperoxide and is, in the further steps (d) and (e)following the step (c), fed to a reaction of propene with hydroperoxide,as follows: (d) the mixture (M2) is admixed with further propene so thatthe propene reacts with most of the unreacted hydroperoxide present inthe mixture (M2) to form propylene oxide, giving a mixture (M4), and (e)the mixture (M4) is returned physically to the separation of step (b).4. The process as claimed in claim 1, wherein the mixture (M2) is fed toa reaction of propene with hydroperoxide in a further step (f) followingstep (c), as follows: (f) the mixture (M2) is worked up if appropriateand returned to step (a).
 5. The process as claimed in claim 1, whereinthe hydroperoxide is hydrogen peroxide and the conversion of hydrogenperoxide in (a) is in the range from 80 to 95%.
 6. The process asclaimed in claim 1, wherein the zeolite catalyst is a titaniumsilicalite catalyst.
 7. The process as claimed in claim 1, wherein thepropene used contains up to 10% by weight of other hydrocarbons.
 8. Aprocess for preparing propylene oxide, which comprises at least thefollowing steps: (a) propene is reacted with a hydroperoxide in asolvent in the presence of a zeolite catalyst to give a mixture (M0),where the mixture (M0) comprises propylene oxide, solvent, unreactedpropene, unreacted hydroperoxide and oxygen, (b) the propylene oxide isseparated from the mixture (M0) so as to give a mixture (M1) whichcomprises unreacted propene and oxygen, (c) the mixture (M1) is broughtinto contact with a liquid medium comprising at least a solvent to givethe mixtures (M2) and (M3), where the mixture (M3) comprises part of theunreacted propene and oxygen and the mixture (M2) comprises solvent,hydroperoxide and residual propene, (d) the mixture (M2) is admixed withfurther propene so that the propene reacts with most of the unreactedhydroperoxide present in the mixture (M2) to form propylene oxide,giving a mixture (M4), and (e) the mixture (M4) is returned physicallyto the separation of step (b), and where the mixture (M3) which has beenseparated off and comprises unreacted propene and oxygen has a ratio ofoxygen to propene such that the mixture (M3) is not ignitable.
 9. Theprocess as claimed in claim 8, wherein the concentration of oxygen inthe mixture (M3) is less than 12% by volume, and wherein the propeneused contains up to 10% by weight of other hydrocarbons.
 10. A processfor preparing propylene oxide, which comprises at least the followingsteps: (a) propene is reacted with a hydroperoxide in a solvent in thepresence of a zeolite catalyst to give a mixture (M0), where the mixture(M0) comprises propylene oxide, solvent, unreacted propene, unreactedhydroperoxide and oxygen, (b) the propylene oxide is separated from themixture (M0) so as to give a mixture (M1) which comprises unreactedpropene and oxygen, (c) the mixture (M1) is brought into contact with aliquid medium comprising at least a solvent to give the mixtures (M2)and (M3), where the mixture (M3) comprises part of the unreacted propeneand oxygen and the mixture (M2) comprises solvent and residual propene,and (d) the mixture (M2) is worked up if appropriate and returned tostep (a), and where the mixture (M3) which has been separated off andcomprises unreacted, propene and oxygen has a ratio of oxygen to propenesuch that the mixture (M3) is not ignitable.
 11. The process as claimedin claim 10, wherein the concentration of oxygen in the mixture (m3) isless than 12% by volume, and wherein the propene used contains up to 10%by weight of other hydrocarbons.
 12. The process as claimed in claim 6,wherein the titanium silicalite catalyst has a TS-1 structure.